1. Field of Invention
The present invention belongs to the field of terahertz technology, and relates to a power measurement apparatus and method for a pulsed terahertz quantum-cascade laser (THz QCL).
2. Description of Related Arts
The terahertz (THz, 1 THz=1012 Hz) radiation is an electromagnetic wave region that is between the millimeter-wave and the infrared light, and covers the frequency range from 100 GHz to 10 THz. With the continuous innovation in the fields of photonics and nanotechnologies, the terahertz radiation is rapidly developed in the fields of information and communication technology, national security, biomedicine, non-destructive testing, quality control of food and agricultural products, and global environmental monitoring, and is considered to have great application prospects and values in these fields. In recent years, the terahertz field developed rapidly, and the THz QCL used as an important radiation source of the terahertz range is subject to extensive and in-depth research, which has achieved important progress. The THz QCL has features such as high energy conversion efficiency, fast response, small size, easy integration, and long life-time. Until now, the highest operating temperature of the THz QCL in a pulsed mode is 186 K, and in an optimal working condition, the highest output power of the device may reach 248 mW. Through structural improvement on an active region of the device, currently the operating temperature of the THz QCL in the pulse operating mode already reaches 1.9 ℏω/kB (ω is a lasing frequency of the laser), and further structural improvement may be expected to enable the device to achieve room temperature lasing. In respect of the lasing frequency, currently the lowest operating frequency of the THz QCL is 1.2 THz, and can reach 0.68 THz under the support of a magnetic field. With the rapid development of the device performance, the application of the device attracts more and more attention. Currently, the THz QCLs have been successfully applied to terahertz technique such as local oscillation sources of heterodyne detection, terahertz wireless communications, and terahertz real-time imaging. The THz QCL in the pulse operating mode requires small cooling capacity and has single pulse peak power being far greater than an output power value of the device in a continuous wave mode, thereby being more competitive in applications of terahertz detection and imaging.
The output power is an important performance indicator for the device application, and the value of the output power directly determines the application field and applied range of the device. Therefore, how to measure the effective output power of the device accurately is a key step in the application process of the device. The repeated frequency of an output laser beam of a pulsed THz QCL is usually about 2 kHz (corresponding to 0.5 ms), but the time constant of a traditional thermal detector (such as Golay Cell) is usually about 20 ms, so that it is very difficult to use the traditional thermal detector to measure the power of the output laser beam of the pulsed THz QCL. Currently around the world, the output power of the pulsed THz QCL is measured mainly through a bolometer detector cooled by using the liquid helium. Integration is performed on a thermal response waveform generated by emitting a single terahertz pulse to the sensitive surface of the detector, so as to estimate energy of a single terahertz pulse, thereby acquiring peak power of the single terahertz pulse. A terahertz quantum-well photodetector (THz QWP) is a semiconductor detector well matching the operating frequency range of the THz QCL. The responsivity of the device to terahertz light in the detectable range may reach the order of magnitude of GHz (the order of magnitude of ns), so that when the THz QWP is used to measure the output power of the pulsed THz QCL, the integration process is not required, and the peak power of pulsed terahertz light output from the THz QCL may be acquired directly according to the amplitude of a response signal and the responsivity of the THz QWP at the lasing frequency of the THz QCL.